Assuming you're running under sufficient permissions, you should be able to set the system-wide date and time by running the
date(1) program. (There is no way to set the time and date on a per-process basis.) This mechanism will work for Unix,
MS-DOS, Windows, and
NT; the
VMS equivalent is
set time.

However, if all you want to do is change your timezone, you can probably
get away with setting an environment variable: